A bearing-free motor with a permanent magnetic rotor is an electromagnetic rotary drive which comprises a permanent magnetically excited rotor and a stator, with the rotor being journalled without contact by means of magnetic forces. The characteristic to which the bearing-free motor owes its name is that it has no separate magnetic bearing for the rotor. For this the stator is designed as a bearing and drive stator and the rotor as a passive magnetic rotor which serves both as a bearing rotor and as a drive rotor. The stator is designed or provided with electrical windings respectively in such a manner that it produces an electromagnetic rotary field which exerts, on the one hand, a torque on the rotor which drives its rotation about the axis of rotation and which, on the other hand, exerts a transverse force on the rotor which can be set in any manner desired so that its radial position with respect to a plane perpendicular to the axis of rotation can be predetermined or actively controlled respectively. Thus in the operating state the rotor can be actively controlled and driven respectively by means of the electric windings of the stator with respect to three degrees of freedom, namely the rotation about the axis of rotation and the radial position in the plane perpendicular to the axis of rotation (two degrees of freedom).
With respect to three further degrees of freedom, namely tiltings with respect to the plane perpendicular to the axis of rotation (two degrees of freedom) and the axial position the rotor is passively magnetically stabilised, that is, not in a controllable manner, by reluctance forces. Thus in the operating state the rotor can be both driven and journalled without contact through the magnetic interaction between the bearing/drive stator and the rotor without separate magnetic bearings being present for this.
The term "bearing-free motor with a permanent magnetic rotor" is to be understood in this sense for the following explanations. With respect to further details of the design and of the excitation and regulation respectively of the bearing-free motor, reference is made here to U.S. Pat. No. 6,100,618.
In U.S. Pat. No. 6,100,618 a bearing-free motor of this kind is disclosed in the example of a rotation pump. In the latter the rotor of the bearing-free motor is provided with vanes and thus forms an integral rotor, which means that it takes over the function of the rotor of the pump in addition to the function of the rotor of the electric motor. Pumps of this kind are advantageous in particular for those uses in which the fluid to be forwarded must not be contaminated, for example for the forwarding of biological liquids such as blood or highly pure liquids such as purest water. In addition rotation pumps of this kind are suitable for the forwarding of aggressive liquids which would destroy mechanical bearings a short time.
In comparison with conventional pumps with a magnetically journalled rotor, pumps of this kind, which operate in accordance with the principle of the bearing-free motor, have the advantage of being extremely compact and space saving and nevertheless having all the advantages of the non-contact magnetic journalling of the rotor even at high performance or forwarding power respectively. This is one of the reasons why pumps of this kind are suitable among other things as blood pumps for uses inside and outside the body.
For the operation of a bearing-free motor with a permanent magnetically excited rotor, in particular for the active regulation of the position of the rotor, which usually takes place by means of a vector regulation method or a field oriented regulation method respectively, it is necessary to know the momentary position of the rotor, that is, its position relative to a plane which is perpendicular to the axis of rotation of the rotor, namely X-Y plane of the stator system. Different possibilities for this are suggested in U.S. Pat. No. 6,100,618. An eddy current distance sensor can be provided in order to measure the distance to a conducting layer which is integrated in the rotor, or optical sensors can be used for the determination of the radial position of the rotor. As further alternatives it is proposed to arrange eight flux probes in the air gap between the rotor and the stator and to determine the radial position of the rotor through weighted summation of the partial fluxes which are measured with the help of the flux probes in each case over half the periphery in the X direction and in the Y direction as well as in each case in the opposite direction, taking of the absolute value and subsequent formation of the difference of the components belonging to the X direction and the opposite direction and, respectively, belonging to the Y direction and the opposite direction.
The present invention deals with the object of proposing another method and another sensor arrangement by means of which the radial position of a permanent magnetic rotor in a bearing-free motor can be reliably determined in a simple manner, economically and with as little cost and complexity as possible.